Interest in catalysis continues to grow in the polyolefin industry. Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. While these catalysts are inexpensive, they exhibit low activity and are generally poor at incorporating α-olefin comonomers. To improve polymer properties, single-site catalysts, in particular metallocenes are beginning to replace Ziegler-Natta catalysts.
With both Ziegler-Natta catalysts and single-site catalysts, it is often an advantage to immobilize the catalyst on a support. This is particularly true for gas-phase, slurry, and bulk monomer processes. Inorganic oxides are known to be useful support materials, but there are often difficulties with catalyst deactivation due to moisture or hydroxyl groups in the inorganic oxide. Typically, the inorganic oxide is treated thermally, chemically, or both prior to use to minimize catalyst deactivation.
For example, U.S. Pat. No. 6,211,311 teaches that many heterometallocenes are inherently unstable and this causes poor catalyst activity and difficulties in supporting these catalysts. This problem is avoided by using chemically treated supports to prepare supported catalysts containing heteroatomic ligands. U.S. Pat. No. 6,559,251 discloses a process for polymerizing olefins with a silica-supported, indenoindolyl Group 4–6 transition metal complex having open architecture. The silica is preferably treated thermally, chemically, or both prior to use. The examples calcine the silica at 250° C. for 12 hours and then treat the silica with an aluminum compound.
While thermal treatment of the inorganic oxide reduces catalyst deactivation, long times and high temperatures are often required. The treatment can be expedited by passing a dry inert gas through the inorganic oxide, but this requires extra equipment and the inorganic oxide can be swept away with the gas. This can be a significant problem for small particle size supports. Chemical treatment is expensive and often requires handling reactive and flammable reagents. A new method for treating inorganic oxides to be used as supports for olefin polymerization catalysts is needed.
Plasma was studied by Langmuir in the early 1900s. More recently, plasma has become widely used in semiconductor fabrication for surface etching and also for plasma-assisted deposition of thin films. If energy is supplied to a gas, it becomes electrically conducting. The energized gas is plasma, which is a mixture of positively charged ions, electrons, and neutral particles. Plasmas have been generated by heating, by applying a voltage, or by irradiation with electromagnetic radiation such as microwaves. Plasma generation by electromagnetic radiation is sometimes referred to as “cold plasma.”
There are several known plasma-treatment processes. For example, U.S. Pat. No. 5,364,519 generates plasma with microwaves for the fabrication of integrated circuits. U.S. Pat. No. 5,647,944 describes a microwave plasma treatment apparatus having a high ashing treatment speed by means of controlling microwaves inside a wave guide tube. U.S. Pat. No. 6,582,778 discloses a method of treating microwave plasma by maintaining a reduced pressure in the plasma-treatment chamber. The patentees use this technique to deposit a film on the surface of a container. U.S. Pat. No. 5,278,384 describes an apparatus that is suitable for a large scale and high capacity. The apparatus generates plasma with an arc torch generator and is used to coat the surface of powder particles.
The use of plasma to oxidize chromium ligands to CrOx was studied for chromium(III) acetate on silica and chromium(III) acetylacetonate on silica to prepare Phillips-type ethylene polymerization catalysts (see J. Phys. Chem. B 101 (1997) 9240). These catalysts, prepared by plasma oxidation, had similar ethylene polymerization activity to those prepared by thermal oxidation of the chromium ligands at 1053° K. In a control experiment, plasma treatment of silica alone showed a loss of water and none of the CO and CO2 generated in the preparation of the chromium catalysts. The authors did not use plasma to treat the silica prior to supporting the transition metal compound. Rather, they used plasma to oxidize the transition metal compound.
Before studying the plasma chemical preparation of chromia supported on zirconia or lanthanum-doped zirconia, the effect of microwave-generated plasma on zirconias without chromium treatment was spectroscopically determined (Catal. Today 89 (2004) 169). A decrease in the number of hydroxyl groups was observed.
New methods for preparing supported olefin polymerization catalysts are needed. Despite the many applications of plasma, apparently no one has used plasma to prepare an olefin polymerization catalyst by contacting an inorganic oxide with plasma prior to supporting a transition metal compound. Plasma contact is as effective as thermal or chemical treatment, but with much shorter times, fewer handling issues, and without the need for hazardous reagents.